in a process of manufacturing a semiconductor device, various films such as a Si3N4 (hereinafter referred to as silicon nitride or SiN) film and so on are formed on a semiconductor wafer (hereinafter referred simply to as a wafer) as a substrate. This film forming process is carried out by a method called ALD (Atomic Layer Deposition), for example, by alternately supplying a precursor gas and a reaction gas onto the wafer multiple times. As a film forming apparatus for performing ALD, an apparatus is known in which a wafer is placed on a rotary table installed in a vacuum container, and the wafer which is revolved by rotation of the rotary table passes repeatedly through an atmosphere to which a precursor gas is supplied and an atmosphere to which a reaction gas is supplied. In addition to supplying the precursor gas atmosphere and the reaction gas atmosphere, there is a case where a modifying gas for modifying a film is supplied onto the rotary table and a plasma generation mechanism, for plasmarizing the reaction gas and the modifying gas in predetermined regions in which the reaction gas and the modifying gas are respectively supplied, is installed.
In the above-mentioned film forming apparatus, since the precursor gas and the gas are also supplied onto the rotary table in addition to the wafer, a film is also formed on the rotary table. Therefore, when a film forming process is not performed, a cleaning gas is supplied into the vacuum container to perform a cleaning process for removing the film on the rotary table. A technique for preventing corrosion of a processing container by a gas containing a halogen element is also known.
In the above-mentioned film forming apparatus, a nitrogen trifluoride (NF3) gas is supplied as the cleaning gas to a region to which the reaction gas and the modifying gas are supplied during the film forming process, and the NF3 gas is converted into plasma by the plasma generation mechanism to carry out cleaning. After completion of the cleaning process, when a SiN film formed with the same process conditions as before the cleaning process, it is found that the film thickness and the film quality of the SiN film are lower than those of a SiN film formed immediately before the cleaning process.
The SiN film formed after the cleaning process contains more fluorine (F) than the SiN film formed immediately before the cleaning process. It is believed that this is because F constituting the plasmarized cleaning gas is doped in the ceiling plate of the vacuum container and is mixed into the SiN film from the ceiling plate during the film forming process. However, a method for coping with the problem that components constituting the cleaning gas remain in the vacuum container in this way is unknown.